Synthesis, Characterization, and Application of Vanadium−Salan Complexes in Oxygen Transfer Reactions

We report the synthesis and characterization of several chiral salen- and salan-type ligands and their vanadium complexes, which are derived from salicylaldehyde or salicylaldehyde derivatives and chiral diamines (1R, 2R-diaminocyclohexane, 1S, 2S-diaminocyclohexane, and 1S, 2S-diphenylethylenediamine). The structures of H (2) sal (R, R-chan) (2+) x 2 Cl (-) x (CH (3) ) (2) CHOH x H (2) O (1c; H (2) sal (R, R-chan) = N, N'-salicyl-R, R-cyclohexanediaminium), Etvan (S, S-chen) (3c; Etvan (S, S-chen) = N, N'-3-ethoxy-salicylidene-S, S-cyclohexanediiminato), and naph (R, R-chen) (6c; naph (R, R-chen) = N, N'-naphthylidene-R, R-cyclohexanediiminato) were determined by single-crystal X-ray diffraction. The corresponding vanadium (IV) complexes and several other new complexes involving different salicylaldehyde-type precursors were prepared and characterized in the solid state and in solution by spectroscopic techniques: UV-vis, circular dichroism, electron paramagnetic resonance, and (51) V NMR, which provide information on the coordination geometry. The salan complexes oxidize in organic solvents to V (V) species, and this process was also studied using spectroscopic techniques. Single crystals suitable for X-ray diffraction were obtained for V (V) O[sal (S, S-dpan) (2) (mu-O) ] x H (2) O x 2 (CH (3) ) (2) CHOH (14c; sal (S, S-dpan) = N, N'-salicyl-S, S-diphenylethylenediaminato) and V (V) O[t-Busal (R, R-chan) (2) (mu-O) ] x 2 (CH (3) ) (2) CHOH (15c), both containing an OV (V) (mu-O) V (V) O moiety (V (2) O (3) (4+) core) with tetradentate ligands and one mu-oxo bridge. Both structures are the first examples of dinuclear vanadium complexes involving the V (V) (2) O (3) (4+) core with tetradentate ligands, the configuration of the V (2) O (3) unit being twist-angular. The V-salen and V-salan complexes are tested as catalysts in the oxidation of styrene, cyclohexene, cumene, and methyl phenyl sulfide with H (2) O (2) and t-BuOOH as oxidants. Overall, the V-salan complexes show higher activity and normally better selectivity in alkene oxidation and higher activity and enantioselectivity for sulfoxidation than their parent V-salen complexes, therefore being an advantageous alternative ligand system for oxidation catalysis. The better performance of V-salan complexes probably results from their significantly higher hydrolytic stability. Mechanisms for the alkene oxidation with these newly obtained V-salan compounds are discussed, including the use of DFT for the comparison of several alternative mechanisms for epoxidation.